Bone Char / Hydroxyapatite

To produce bone char suitable for use as a filter material, animal bones are charred in a kiln at a defined temperature and oxygen content - in this way the specific surface area of the bones is increased, organic constituents are removed and the inorganic hydroxyapatite matrix remains (Ca₅(PO₄)₃OH). The charred bones are sieved and crushed to produce a granular filter material. Fluoride removal with bone char is based on an adsorption process. The fluoride ions bind to surface sites on the hydroxyapatite, thereby releasing OH^- into solution.

Ca₅(PO₄)₃OH + F^- ↔ Ca₅(PO₄)₃F + OH^-

Once the uptake capacity has been reached and most surface sites are occupied, the ability to bind fluoride decreases rapidly. At this point the filter material needs to be regenerated or replaced.

Advantages

• locally available, cheap raw material (animal bones)
  
• high sorption capacity
• fast reaction time (less than 30 min)
• no additional chemicals are needed for the treatment process
• filtered water is neutral in taste and colour
  
  

Disadvantages

• limited acceptability of animal bones as filter material for drinking water
• initial investments and experience needed to set up bone char production
  
• maintenance and regular monitoring necessary - if filter is saturated it needs to be regenerated or replaced
  
  

Synthetic hydroxyapatite has a higher surface area and adsoprtion capacity than bone char and is generally more effective in removing fluoride. As it needs to be imported it is more expensive than the locally available bone char and therefore often not an option for projects in developing countries.

 

Activated Alumina

Nalgonda Technique

Contact Precipitation

Reverse Osmosis